High-frequency electrostatically shielded toroidal plasma and radical source

ABSTRACT

An electrostatically shielded toroidal plasma and radical source is provided. The plasma source includes a grounded metallic plasma source chamber that defines an interior for plasma generation. The plasma source chamber is configured from two L-shaped portions arranged to form rectangularly shaped enclosure. Dielectric breaks are defined by gaps between the two L-shaped portions. A drive inductor is configured such that the metallic plasma source chamber is positioned between loops of the drive inductor.

BACKGROUND OF THE INVENTION

[0001] The present invention is related to substrate processingequipment and more particularly to plasma processing equipment forperforming plasma processing steps such as deposition, cleaning, and/oretch processes on a process substrate.

[0002] It is well known that plasma discharges may be used to excitegases to produce activated gases containing ions, free radicals, atoms,and molecules. Such activated gases are used for numerous industrialapplications, including, in particular, various operations performedduring the fabrication of semiconductor devices. For example,plasma-processing methods are used in deposition processes, such asplasma-enhanced chemical vapor deposition (PECVD) or high-density-plasmachemical vapor deposition (HDP-CVD), to deposit layers of material onsubstrates. Plasma-processing methods are also used within a number ofetching techniques, such as reactive ion etching (RIE) or deep RIE(DRIE). Plasmas are also used in cleaning processes to prepare aprocessing chamber or the surface of a particular substrate forsubsequent processes; such processes include a plasma wafer surfaceclean or activation prior to formation of a layer on the surface.

[0003] Generally, plasma-processing applications can be characterized bythe kinetic energy of the ions in the plasma and by the level of directexposure the material being processed has to the plasma. For example,applications sensitive to material damage generally requirelow-kinetic-energy ions and/or shielding of the material from theplasma, while applications such as anisotropic etching require ions withhigh kinetic energy. Certain applications, such as RIE or DRIE requirerelatively precise control of the ion energy. Applications such asgenerating ion-activated chemical reactions, and etching or depositionof material into high-aspect-ratio structures, are examples of processesthat make use of direct exposure of the material to a high-densityplasma.

[0004] This wide application of plasma processing uses is reflected inthe extensive variety of available plasma processing systems andapparatuses. The basic methods these systems use for plasma generationinclude dc discharge, RF discharge, and microwave discharge. Oneparticular type of plasma processing chamber places the wafer on anelectrode of the plasma circuit, opposite another planar electrode, andcapacitively couples high-frequency electrical power to the twoelectrodes to form a plasma between them. Such a plasma reactor hasadvantages where it is desirable to form the plasma in the presence ofthe substrate, such as when the physical movement of plasma species toand from the substrate is specifically desired. However, some devices ormaterials are not readily compatible with this type of plasma formation,particularly because the plasma includes high-energy photons and theirdirect bombardment on the substrate results in undesirable heating.Another approach to plasma processing generates plasma in a remotelocation and couples the plasma to a processing chamber. Various typesof remote plasma generators have been developed, including magnetronsources coupled to a cavity, inductively coupled toroidal sources,microwave irradiation directed at a plasma precursor, electron-cyclotronresonance generators, and others. For particular types of processes,such as cleaning processes, remote plasma techniques offer certainadvantages.

[0005] Inductively coupled RF plasma systems are often used inprocessing semiconductor wafers, in part because they can generatelarge-area plasmas. In principle, inductively coupled plasma systemspermit generation of a high-density plasma in one portion of aprocessing chamber (e.g. above the material being processed) andsimultaneous shielding of the material from the plasma-generationregion. Such systems attempt to use the plasma itself as a protectivebuffer that protects the material from various possible deleteriousplasma effects attributable to characteristics of the plasma-generationregion. Because the drive currents are only weakly coupled to theplasma, however, these plasmas cannot be made absolutely inductive andrequire high voltages on drive coils to compensate for the resultinginefficiency. These high voltages produce large electrostatic fieldsthat cause high-energy ion bombardment, primarily on the reactorsurfaces, but also on the material being processed.

[0006] Approaches to shield the electrostatic fields have includedpositioning Faraday shields within the process chamber, but the weakplasma-drive-current coupling results in the formation of large eddycurrents in the shields, which in turn produces substantial powerdissipation. An alternative approach, such as described in WO 99/00823,entitled “TOROIDAL LOW-FIELD REACTIVE GAS SOURCE,” incorporated hereinby reference, attempts to exploit a specific transformer arrangement ina toroidal RF plasma source. Semiconductor switching devices are used todrive the primary winding of a power transformer that coupleselectromagnetic energy to the plasma, thereby forming a secondarycircuit of the transformer.

[0007] Toroidal plasma-source devices such as that described in WO99/00823 have a number of limitations that it is desirable to overcome.For example, they are typically designed for only a specific load,thereby having limited operational flexibility.

[0008] They are, moreover, restricted to operation at low RF frequencies(typically about 400 kHz), and require the use of a magnetic core, whichcontributes to efficiency losses. They also require an auxiliary starterto initiate plasma formation and require a flow of inert gas, such asAr, to maintain the plasma. Such limitations are overcome with thepresent invention.

SUMMARY OF THE INVENTION

[0009] Embodiments of the invention are directed to an electrostaticallyshielded toroidal plasma source that does not use a magnetic core.Instead, the operation of the plasma source is achieved by directinductive coupling between a current in a driving coil with the plasmacurrent in the plasma chamber. The toroidal plasma source according toembodiments of the invention can be operated at high RF frequencies,i.e. greater than 400 kHz, with only water cooling. Plasma formation isachieved without the need for an auxiliary starter and without the needfor including a flow of inert gas. The toroidal plasma source canaccordingly be configured with a substrate processing system to achieveimproved overall efficiency.

[0010] In a first embodiment, a metallic plasma source chamber definesan interior for plasma generation. The plasma source chamber includes atleast one dielectric break. A drive inductor is configured such that themetallic plasma source chamber is positioned between loops of the driveinductor. An input coil is configured proximate the drive inductor toprovide a mutual inductance between the input coil and the driveinductor. In one embodiment, the plasma source chamber is configuredfrom two L-shaped portions assembled to form a rectangularly shapedenclosure. The dielectric break is defined by a gap between the twoL-shaped portions. In one embodiment, the metallic plasma source chamberis grounded.

[0011] In another embodiment, the interior of the plasma source chamberis lined with a material that can be heated by the plasma, such asquartz. The liner acts to reduce losses due to oxygen recombination onsurfaces, thereby improving the efficiency of substrate-processingoperations.

[0012] These and other embodiments of the present invention, as well asits advantages and features are described in more detail in conjunctionwith the text below and the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a simplified schematic diagram of a plasma-basedchemical-vapor-deposition system according to an embodiment of thepresent invention;

[0014]FIG. 2(a) is a perspective illustration of one embodiment of theplasma source chamber according to the present invention;

[0015]FIG. 2(b) shows a cross-sectional view of the plasma sourcechamber, showing the positioning of centering rings used as dielectricbreaks in one embodiment;

[0016]FIG. 2(c) shows a perspective illustration of the plasma sourcechamber configured to act as a downstream plasma source.

[0017]FIG. 3 is an equivalent circuit diagram showing the electricalcharacteristics of a toroidal plasma source according to an embodimentof the invention in operation;

[0018]FIG. 4 generally is a schematic illustration of differentarrangements that may be used with the input loop and drive inductor toadjust their mutual inductance:

[0019]FIG. 4(a) shows an embodiment where the axes of the input loop anddrive inductor are parallel; FIG. 4(b) shows an embodiment where theaxes of the input loop and drive inductor are perpendicular; FIG. 4(c)shows an embodiment where the axes of the input loop and drive inductorare at an intermediate angle; FIG. 4(d) shows an embodiment where ametal strip is positioned between the input loop and drive inductor;

[0020]FIG. 5 is a graphical representation of general arc-discharge andgas-breakdown behavior;

[0021]FIG. 6(a) shows one configuration of an open plasma source chamberin accordance with the invention;

[0022]FIG. 6(b) shows one configuration of a multiple-flow-port plasmasource chamber in accordance with the invention; and

[0023]FIG. 6(d) shows an embodiment in which multiple plasma sourcechambers are constructively configured to increase overall flow.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0024] I. Introduction

[0025] Embodiments of the present invention are directed to a downstreamtoroidal plasma source and a distributed plasma source that may be usedas part of a semiconductor processing system. The plasma source may beconfigured to provide an ionized plasma and to provide a source ofradicals; accordingly, the phrase “plasma source” is used herein torefer inclusively to a source for an ionized plasma and/or radicals. Asdescribed in detail below, embodiments of the invention include anelectrostatically shielded plasma source that may run with 100% oxygenundiluted by an inert gas and with a nonmagnetic core. Tests on a devicefabricated in accordance with the invention and with these propertiesshow that it achieves substantially greater efficiency than priordevices that require use of a magnetic core.

[0026] II. Exemplary Substrate Processing System

[0027] The toroidal plasma source of the present invention may be usedwith a substrate processing system such as that shown schematically inFIG. 1. The substrate processing system 10 may be used for a variety ofplasma processes, including plasma-based deposition processes andplasma-etching processes. The substrate processing system 10 includes aprocess chamber 12 having a chamber body 14, a vacuum system 18, a biasplasma system 30, a gas-delivery system 36, a system controller 44, anoptional remote-plasma cleaning system 104, and a downstream plasmasource system 124.

[0028] The process chamber 12 includes a substrate support member 74positioned within the process chamber 12 to hold the substrate 32 duringprocessing. The substrate support member 74 is configured to supportwafers, which may, for example, have a diameter of approximately 200 mmor 300 mm for an appropriately sized process chamber 12. A bias plasmasystem 30 is optionally included for creating a potential difference atthe substrate support member 74 to produce electrodynamic movement ofthe plasma normal to the substrate 32.

[0029] The gas delivery system 36 provides gases to the process chamber12 and other system components through gas delivery lines 38, only someof which may be shown explicitly in FIG. 1. Typical gases provided bythe gas delivery system 36 might include plasma precursor gases, such asa cleaning or etching plasma precursor gas, a plasma depositionprecursor gas, plasma striking gas, plasma dilution gas, and othergases, such as a cleaning precursor gas provided to an optional remoteplasma cleaning system 104, for example. The delivery lines 38 generallyinclude some sort of control, such as a mass-flow controller 42 andshut-off valves (not shown). The timing and rate of flow of the variousgases is controlled through a system controller 44, described in greaterdetail below.

[0030] Substrates are transferred into and out of the process chamber 12by a robot blade (not shown) through an insertion/removal opening (notshown) in the side of the chamber body 14. Motor-controlled lift pins(not shown) are raised and then lowered to transfer the substrate 32from the robot blade to the substrate support member 74. The substratesupport member 74 may include a wafer-hold-down apparatus, such as anelectrostatic chuck (not shown), that can selectively secure thesubstrate 32 to the substrate support member 74 during substrateprocessing if desired. In certain embodiments, the substrate supportmember 74 is made from anodized aluminum, aluminum, or aluminum oxide.

[0031] The temperature of the wafer may be controlled in differentembodiments. For example, the substrate support member 74 may include aheater (not shown) to heat the wafer during processing, or to heatportions of the process chamber 12 during a cleaning process.Alternatively, a heat-transfer gas, such as helium (He), may be flowedthrough inner and/or outer passages in the wafer chuck. The gas flow hasthe additional effect of thermally coupling the substrate to the chuck.In a typical process, the wafer is heated by the plasma and the chemicalreactions that form the layer, and the He cools the substrate throughthe chuck, which may be water-cooled. This keeps the substrate below atemperature that may damage preexisting features on the substrate.

[0032] The vacuum system 18 includes throttle body 76, which housestwin-blade throttle valve 78 and is attached to gate valve 80 andturbo-molecular pump 82. It should be noted that throttle body 76 offersminimum obstruction to gas flow, and allows symmetric pumping, asdescribed in commonly assigned U.S. patent application Ser. No.08/712,724 entitled “SYMMETRIC CHAMBER,” by Ishikawa, filed Sept. 11,1996, and which is herein incorporated by reference for all purposes.The gate valve 80 can isolate the turbo-molecular pump 82 from thethrottle body 76, and can also control chamber pressure by restrictingthe exhaust flow capacity when the throttle valve 78 is fully open. Thearrangement of the throttle valve 78, gate valve 80, and turbo-molecularpump 82 allows accurate and stable control of chamber pressures betweenabout 1 millitorr and 3 torr. It is understood that other types ofvacuum pumps and configurations of vacuum systems could be used withalternative embodiments of the present invention.

[0033] The bias plasma system 30 includes a bias generator 86 and anoptional bias matching network 88. The bias plasma system capacitivelycouples the substrate support member 74 (and therefore also thesubstrate) to conductive (grounded) inner surfaces of the chamberthrough a common ground 90. The bias plasma system 30 serves to enhancethe transport of plasma species, including reactive ions and otherparticles, created at the plasma source chamber to a surface of thesubstrate 32. The plasma source chamber 100 is also grounded throughcommon ground 90.

[0034] The gas delivery system 36 provides gases from several gassources 92, 94, 96, and 98 to the chamber and other system componentsvia the gas delivery lines 38, only some of which might be shown. Gasescan be introduced to various components of the substrate processingsystem in a variety of fashions. For example, gases can be introducedinto the process chamber 12 through a side port 70, as shown, or througha top port 71. A gas mixing chamber (not shown) can be present betweenthe gas sources and the chamber, or the top and/or side ports can bearranged with a number of parallel or concentric gas conduits to keepvarious gases separate until reaching the chamber. In an alternativeembodiment, a gas delivery ring with a series of gas nozzles is providedabout an inner circumference of the processing chamber.

[0035] The optional remote plasma cleaning system 104 is provided forperiodic cleaning of deposition residues from chamber components. Thecleaning system includes a remote microwave generator 106 that creates aplasma from a cleaning gas source 98 such as molecular fluorine,nitrogen trifluoride, other fluorocarbons or equivalents, in a reactorcavity 108. The reactive species resulting from this plasma are conveyedto the interior of process chamber 12 through a cleaning-gas feed portvia an applicator tube 112.

[0036] The downstream plasma source system 124 includes an RF generator(power supply) 20 coupled to a single-turn input loop 27 by leads 24 and26. The RF power is mutually coupled from the single-turn input loop 27to a two-turn drive inductor 28 driven by drive 22 and resonated with acapacitor 29. The plasma source chamber 100 is positioned between theturns of the two-turn drive inductor and operates to provide a plasma tothe process chamber 12 through applicator tube 122. The drive inductor28 may generally be of any shape provided sufficient mutual flux linkageexists between the current-carrying drive inductor 28 and the interiorof the plasma source chamber 100. In different embodiments, additionalcoil turns may be included in the drive inductor 28 on either side ofthe plasma source chamber 100. This design uses direct coupling betweenthe drive inductor 28 and the plasma current in the plasma sourcechamber 100 so that a magnetic core is unneeded. Sometimes the designmay conveniently be referred to as having an “air core” to distinguishit from magnetic-core designs, although strictly the design is moreproperly characterized as relying on direct coupling without a core.

[0037] The RF generator 44 operates at a nominal frequency of 13.56 MHz,but could operate at different frequencies, such as 60 Hz, 400 kHz, 2MHz, 60 MHz, or 200 MHz among others, with appropriate design of theelements of the plasma system. The RF generator can supply up to 8 kW ofpower, but the processing system typically draws about 3-5 kW whenprocessing a 200 mm wafer. It is understood that higher or lower powerlevels might be appropriate according to the type of process beingperformed and the size of the substrate.

[0038] The specific embodiment shown in FIG. 1 is not intended to limitthe invention. For example, while the embodiment shown in FIG. 1illustrates a configuration of the downstream plasma source system 124that uses a single-turn input loop 27 mutually coupled with a two-turndrive inductor 28, other configurations may use different numbers ofturns for these components. This electrical structure for the downstreamplasma source system 124 is described more fully in FIG. 3 and relatedtext below. While FIG. 1 illustrates schematically an embodiment wherethe plasma source chamber 100 is configured as a remote plasma source toprovide a plasma at the top of the process chamber 12 through anapplicator tube 122, the invention is not so limited, and it mayalternatively be configured to provide the plasma at other locations ofthe process chamber 12.

[0039] The system controller 44 controls the operation of the substrateprocessing system 10. In one embodiment, the system controller includesa processor 114 coupled to a memory 116, such as a hard disk drive, afloppy disk drive, and a card rack (not shown). The card rack maycontain a single-board computer (not shown), analog and digitalinput/output boards (not shown), interface boards (not shown), andstepper motor controller boards (not shown). The system controller 44 iscoupled to other parts of the processing system by control lines 118(only some of which might be shown), which may include system controlsignals from the system controller 44 and feedback signals from thesubstrate processing system 10. The system controller 44 conforms to theVersa Modular European (VME) standard, which defines board, card cage,and connector dimensions and types. The VME standard also defines thebus structure having a 16-bit data bus and 24-bit address bus. Systemcontroller 44 operates under the control of a computer program 119stored on the hard disk drive or other computer programs, such asprograms stored on a floppy disk. The computer program dictates, forexample, the timing, mixture of gases, RF power levels, and otherparameters of a particular process. The interface between a user and thesystem controller 44 is via a monitor (not shown), such as a cathode raytube, and a light pen (not shown).

[0040] The toroidal plasma source according to various embodiments ofthe present invention may be used in numerous substrate-processingapplications, in addition to plasma-based deposition procedures. Forexample, if the precursor gases include fluorine sources, such as NF₃ orSF₆, it may be used to provide a plasma for downstream cleaning. Theplasma source may alternatively be used to provide an etching source,including for polymer etching and photoresist stripping. It is alsospecifically understood that other types of chambers might be adapted toa toroidal plasma source according to the present invention, and thatdifferent types of wafer support systems, such as a center pedestal,might be used, as well as different exhaust configurations, such as aperimeter exhaust configuration.

[0041] III. Downstream Toroidal Plasma Source

[0042] The general configuration of the plasma source chamber 100according to one embodiment of the invention is illustrated in FIG. 2.Generally, the plasma source chamber 100 comprises a metallic enclosurethat defines an internal closed-loop path for circulation of plasmaspecies. The chamber includes at least one dc break gap which may beconfigured as described below to optimize operational conditions for theplasma source by balancing arc-discharge and gas-breakdowncharacteristics. In certain embodiments, fabrication of the plasmasource chamber 100 is facilitated by assembling it from two or moreindividual components. In such cases, the number of dc break gaps willbe equal to the number of component elements. One or more of theresulting dc break gaps may be conductively shunted with a metallicstrip, permitting definition of the operational conditions in terms of asingle nonshunted gap.

[0043] The specific plasma source chamber 100 shown in FIG. 2(a)provides an example of an embodiment where a plurality of individualcomponents are used. In this example, two L-shaped metallic pieces 110and 120 are assembled to form an substantially rectangularly shapedchamber with an approximately 1-inch diameter bore forming the closedpath. In one embodiment, both L-shaped pieces 110 and 120 are formedfrom aluminum, either anodized or unanodized, permitting the plasmasource chamber 100 to be water-cooled. Alternative cooling fluids, suchas air, nitrogen, gas or helium gas may also be used, but as describedbelow, the plasma source chamber 100 does not require active cooling.The cooling fluid can be provided through a conduit configuration inthermal communication with the plasma source chamber 100. Cooling finsto increase the total area of thermal communication with plasma sourcechamber 100 could also be added. In alternative embodiments, othermetals that do not require active cooling, such as copper, are used toform the plasma source chamber. Gases are input into the chamber, andplasma in output from the chamber through flow ports 130, one of whichis shown in FIG. 2(a). In different embodiments described below, theplasma source chamber 100 may be equipped with a plurality of flow ports130 depending on how the plasma source chamber 100 is configured withrespect to the process chamber 12.

[0044] The rectangular configuration of the plasma source chamber 100 inthe exemplary embodiment defines two gaps 115 when the two L-shapedpieces 110 and 120 are configured to form a rectangle. Dielectric breaks117, such as shown in FIG. 2(b), in the gaps 115 prevent electricalshorting of the individual source chamber components between each otherand do not impede the penetration of RF induction fields into the vacuumregion within the plasma source chamber 100. The dielectric breaks 117may be formed, for example, with teflon centering rings (such as, e.g.,KF-25 teflon centering rings), as shown in the cross-sectional view of ajunction between the two component pieces in FIG. 2(b). The dielectricbreaks 117 are positioned within a gap 115, which may itself be definedmore particularly by shaping the L-shaped pieces 110 and 120 forinterconnection at the junction, for example as shown in FIG. 2(b).

[0045] With the configuration of two L-shaped pieces 110 and 120 shownin FIG. 2(a), the plasma source chamber 100 may have two gaps 115 at thejunctions of the individual components or may intentionally beelectrically shorted at one of the gaps, for example by includingmetallic (aluminum) shunt 140, so that operationally the plasma sourcechamber 100 functions as with a single gap 115. While the rectangularembodiment comprising two L-shaped components is convenient tofabricate, there are alternative configurations that are also within thescope of the invention. For example, the junctures of the legs could bearcuate. Alternatively, the shape of the plasma source chamber 100 maydefine a nonrectangular polygon or a continuous closed curve such as acircle or ellipse.

[0046] In operation, the plasma source chamber 100 is placed between theturns of the two-turn drive inductor, which is also preferablycapacitively resonated. The drive inductor may be comprised of widesheet metal for low loss and tight magnetic coupling to the plasmasource chamber 100. The RF power is mutually coupled from a single turninput loop 27 to the two-turn drive inductor. A resulting advantage ofthe invention is that with such a configuration no magnetic core isneeded.

[0047] One embodiment in which the plasma source chamber 100 isconfigured as a downstream plasma source is shown in detail in FIG.2(c). In this embodiment, using the rectangular configuration, gasenters at one corner of the rectangular plasma source chamber 100 andplasma and/or active neutrals leave from the diagonally opposite corner.As shown, gas is provided to the plasma source chamber 100 though a gasdelivery line 134, which is connected to the flow port with a gas-tightconnector 132. For appropriate RF energy, the penetration of the RFinduction fields into the plasma source chamber 100 provides sufficientenergy to ionize the gas to form and maintain the plasma. No auxiliarystarting mechanism is required for plasma initiation and there is nooperational distinction between initiation and load conditions.

[0048] This plasma initiation behavior has been confirmed with specificobservations through observation windows, which may optionally beincluded on sides of one or both of the L-shaped pieces 110 or 120.During such observations, a brief transition of capacitive discharge maybe observed when both precursor gas and RF induction fields areinitially present in the plasma source chamber 100, but there issufficient capacitance field at the gap(s) 115 that the plasma initiatesspontaneously when operational conditions are satisfied. As a result,there is no significant transition between the initiation state and thesteady state.

[0049] As the gas is ionized to form the plasma along path 135, plasmaspecies and/or active neutrals leave the plasma source chamber 100through applicator tube 122. The applicator tube 122 connects the plasmasource chamber 100 to the process chamber 12 with gas-tight connectors136 and 138. Such a configuration is suitable, for example, forapplications such as downstream etching.

[0050] The operational characteristics of the plasma source according tothe invention may be further understood with reference to FIG. 3, whichshows a circuit diagram equivalent to the electrical behavior governingoperation of the plasma source. The RF generator 20 is in electricalcommunication with the single-turn input loop 27 having a variable inputinductance L_(input). The plasma source chamber 100 itself has a chamberinductance L_(chamber) and capacitance C_(gap), which is determined bythe size of gap(s) 115 between the L-shaped components. The inductanceof the two-turn drive inductor between whose turns the plasma sourcechamber 100 is placed is denoted by L_(coil) with the variableresonating capacitance denoted by C_(tune). The circulation of theionized plasma particles within the plasma source chamber 100 produces afurther inductance L_(plasma), the plasma also having a resistivecomponent R_(plasma). The coupling of these inductive components forms atransformer circuit that operates as part of the toroidal plasma sourcewhen the process chamber is in operation. As indicated, the plasmasource chamber 100 is grounded through common ground 90, such that noadditional dc gap is required.

[0051] For this example circuit, the plasma source is matched to the RFgenerator by varying two things: (1) the coefficient of coupling K (andtherefore the mutual inductance) and (2) resonating with the tunecapacitance C_(tune). The coupling K may be varied by changing theproximity between the input loop 27 and drive inductor 28, oralternatively by changing the rotational orientation between the inputloop 27 and drive inductor. For example, as shown in FIG. 4(a), the axesof the input loop 27 and drive inductor 28 may be oriented parallel toone another; they may be oriented perpendicularly to one another asshown in FIG. 4(b); or they may be oriented at intermediate positions asshown in FIG. 4(c). Alternatively, the coupling K may be varied byinserting one or more metallic blades 25 between the input loop 27 anddrive inductor 28 as shown in FIG. 4(d).

[0052] While the input loop self-inductance L_(input) may beincidentally affected by such variations, the change in K is much moresignificant.

[0053] The use of direct coupling between the drive inductor and theplasma current according to the invention may be adapted to othersubstrate processing systems. For example, a toroidal plasma sourcehaving an “air core” may be incorporated within the process chamber 12.An illustration of a toroidal plasma source incorporated within theprocess chamber 12 is described in the copending, commonly assigned U.S.Patent Application, filed May 25, 200 and assigned Ser. No. 09/584,167,entitled “TOROIDAL PLASMA SOURCE FOR PLASMA PROCESSING,” by Michael S.Cox et al., which in incorporated herein by reference for all purposes.

[0054] IV. Operating Parameters

[0055] Considerations used to determine the operational characteristicsof the toroidal plasma source, including the size of the gap(s) 115, areillustrated in FIG. 5, which plots the general arc-discharge andgas-breakdown behaviors. The curves are plotted in logarithm-logarithmform. The logarithm of the arc-discharge voltage increases monotonicallywith the logarithm of the product Pd_(gap), where P is the pressure inthe plasma source chamber 100 and d_(gap) is the size of the gap 115.The gas-breakdown curve includes a characteristic minimum atPd_(gap)≈0.5 torr cm and V≈300 V. In many cases, it is desirable tominimize capacitive coupling and maximize inductive coupling of RF powerfrom generator to plasma for maximum reaction rate (etch rate ordeposition rate, for example). Although the source is electrostaticallyshielded by grounding the plasma source chamber halves, the existence ofan inductively coupled plasma in the plasma source chamber requires aninduced voltage along the plasma within the source plasma chamber.

[0056] Typical induced electric field magnitudes are a few volts per cm(i.e. 2-4 V/cm). For a path length of about 60 cm, this gives rise to120 to 240 volts induced loop voltage. This voltage appears across thedc break gap(s). Depending on this induced loop voltage, the gaspressure within the source chamber and the effective gap(s) distance,there may be a capacitively coupled plasma proximate to the gap(s). Tominimize the capacitive coupling, the gap may be selected to avoid theregion of the gas breakdown curve that is near the minimum (where theminimum voltage can break down the gas).

[0057] The gap may be selected to operate on the left-hand-side of theminimum (small Pd_(gap) product relative to the minimum Pd_(gap) foreasiest gas breakdown) or on the right-hand-side of the minimum (largePd_(gap) product relative to the minimum Pd_(gap) for easiest gasbreakdown). For left-hand-side operation, and 2× margin (of Pd_(gap))from the Pd_(gap) minimum, the condition Pd_(gap)≦0.25 torr cm should besatisfied. Thus (for left-hand-side operation), for example, at 20 torr,the gap should be ≦0.125 mm, and at 0.2 torr, the gap should be ≦12.5mm. For right-hand-side operation, and 2× margin (of Pd_(gap)) from thePd_(gap) minimum, the condition Pd_(gap)≧1 torr cm should be satisfied.Thus (for right-hand-side operation), for example, at 20 torr, the gapshould be ≧0.5 mm, and at 0.2 torr, the gap should be ≧50 mm.

[0058] Taking the left-hand-side solution for a high pressure of 20torr, the gap should be ≈0.125 mm. Using the left-hand-side solution mayleave the possibility of an undesirable arc discharge at the gap(s).This can be minimized by coating the metal surfaces in the gap area witha sufficiently high-dielectric-strength insulator such as by anodizationor by using a high-dielectric-strength solid insulator between metalsurfaces (such as Al₂O₃). Finally, the capacitive reactance per unitlength across the gap should be large relative to the impedance per unitlength of the plasma loop to avoid capacitively shorting out the plasmasource chamber halves, which could shield out the RF inductive field.

[0059] For a plasma loop impedance of the order of 1Ω and a loop lengthof 60 cm, then the capacitive reactance per unit length across the gapshould be large compared to 1Ω/60 cm. For a plasma source chambercross-section of 4 cm×4 cm, with a 2.5 cm diameter bore, the area isA=11 cm². For a gap of 0.125 mm and unit dielectric constant k=1, thenthe capacitance C across the gap is approximately 79 pF, as determinedfrom the relationship C=Aε₀k/d_(gap), where ε₀ is the permittivity offree space. At an RF frequency of 13.56 MHz, the impedance of the gap isthus approximately 149Ω, as determined from the relationship Z=1/ωC,with ω equal to 2π times the RF frequency. The impedance per unit lengthof the gap is thus 149Ω/0.125 mm=1.2×10⁴ Ω/cm. This is significantlylarger than the impedance per unit length of the plasma loop of 1Ω/60cm=0.02Ω/cm. Thus, in the absence of a strong capacitive plasma at thegap (precluded by appropriate selection of Pd_(gap) as described aboveto avoid the Pd_(gap) minimum), the capacitive reactance across thegap(s) will not short out the plasma source chamber and preventinductive coupling to it.

[0060] As described above, the effective transformer arrangement usesdirect coupling between the driving inductor and the plasma current. Asa result, losses attributable to the use of a magnetic core, such asferrite, used in other toroidal plasma source designs are avoided.Thermal considerations that govern the invention permit operationwithout requiring a magnetic core, so that the known coolingcomplexities associated with the use of magnetic cores in toroidalplasma sources are thus also avoided. For example, incontinuous-operation tests of a plasma source constructed according tothe embodiment of the invention described above, at a power of 2.5 kWfor more than 30 minutes, no thermal runaway was observed and theexterior temperature of the plasma source chamber 100 did not exceed 40°C. even without a magnetic core. During such tests, operation of theplasma source in an inductive mode was confirmed through directobservation of a continuous tube of plasma emission within the plasmasource chamber 100. Such observations were specifically contrasted withcapacitive discharge, which instead would have shown a maximum emissionbetween the L-shaped chamber halves. Losses in the input loop and drivecoil were low, as indicated by the lack of significant temperature riseeven with only air cooling of the coils.

[0061] The plasma source chamber 100 according to the embodimentdescribed above effectively accommodates mass flow rates of 0.5-20liters/min of oxygen. This oxygen-delivery capability has a notableeffect on the operational characteristics of the plasma source when usedto strip photoresist. Molecular oxygen molecules provided to the plasmasource chamber 100 ionize to form oxygen molecular ions O₂ ⁺. Themolecular ions dissociatively recombine with electrons to from twooxygen atoms, which are an effective photoresist etching agent. It isthus desirable to have a plasma source system that can accommodategreater oxygen flow rates.

[0062] Photoresist-stripping tests have been performed to compare theseoperational characteristics with those of the magnetic-core devicedescribed in WO 99/00823 (“the '823 device”). Photoresist silicon waferpieces with an area of ˜1 cm² were clamped to a temperature-controlledsubstrate support member 74 about 20 cm downstream of the plasma source.Using a pressure{tilde under (>)}0.5 torr, no plasma is present that farfrom the source outlet. Typically, downstream emission (a dim greencolor) is present below the plasma source in the region of the substratesupport member 74. When polymer is etching a sufficient rate, a thinblue layer of emission is visible at the surface of the sample. Bothplasma sources provided by the present invention and by the '823 devicetransfer significant heat to the neutral gas at high mass flow rates.

[0063] In the test trials, the '823 device could not be operated with100% oxygen and required dilution with, e.g., Ar to sustain the plasma.The '823 device operated with a 360 kHz switch-mode generator with fixedtransformer impedance ratio and fixed 300 V (dc) line voltage. Thephotoresist strip rate was optimized by flowing as much oxygen aspossible (4 liters/min) while maintaining plasma, using high Ar flowdilution (10 liters/min) and maximum pumping speed (pressure of 3 torrin the process chamber 12). Under these conditions, the RF powerdelivered was 5 kW. With the substrate support member 74 maintained at80° C., the photoresist etch rate was a repeatable 3500 Å/min.

[0064] The toroidal plasma source shown in FIG. 3, which may instead beoperated with 100% oxygen, was run at an RF frequency of 13.56 MHz. Theetch rate was optimized by using an oxygen flow rate of 9 liters/min atmaximum pumping speed (pressure of 2 torr in the process chamber 12).With RF power also delivered at 5 kW and the substrate support member 74also maintained at 80° C., the photoresist etch rate was a repeatable7500 Å/min. As explained above, the higher etch rate realized by theplasma source according to the present invention is a consequence of theability to maintain a plasma at significantly greater (undiluted) oxygenflow rates.

[0065] V. Interior Liner

[0066] It has been discovered by the inventors that still furtherincreases in etch rates may be achieved by lining the interior of theplasma source chamber 100 with a material that is heated with energyfrom the plasma. For example, when the downstream plasma source is usedto provide an etchant, including such a liner made of quartz is observedto produce an increase in photoresist strip rates of ˜4 μm/min. Underthe operating conditions described above, heat generated by the plasmawithin the plasma source chamber causes the quartz liner to reach atemperature of approximately 600-700° C.

[0067] It is hypothesized that the heat of the liner acts to reducelosses due to recombination of oxygen on the interior surfaces of theplasma source chamber 100. This mechanism may operate to produce asynergistic enhancement of photoresist etch rates as a result oftemperature and material effects. Alternatively to quartz, the liner maybe manufactured of ceramic, Si, SiC, Al₂O₃, or sapphire, among othermaterials, and still achieve an improvement in etch rates over anunlined plasma source chamber.

[0068] VI. Exemplary Distributed Plasma Source Chamber Configurations

[0069] The plasma source chamber 100 according to the present inventionmay also be used in configurations as a distributed plasma source.Various such exemplary configurations are illustrated in FIGS.6(a)-6(c), although other configurations are also within the scope ofthe invention. For simplicity, the drive inductor(s) 28 is not shown.The plasma course chamber 100 lends itself to several differentplasma-movement mechanisms that may be adopted, including diffusion andbias-initiated flow.

[0070] One embodiment is shown in FIG. 6(a), where the basic closeddouble-L-shaped structure is modified by removing a portion of thestructure to produce an open plasma source chamber 100′. In theillustrated embodiment, the open plasma source chamber 100′ includes asingle gap 115′. Precursor gases are provided to the open plasma sourcechamber 100′ with gas-delivery line 134, which is connected to the openplasma source chamber 100′ with gas-tight connector 132. Plasma movementmay be directed with an induction coil 158, which creates an electricfield within the open plasma source chamber 100′ to act on ionicspecies. The charged particles thus follow a closed path 162, whichcauses the plasma species to move proximate the substrate support member74. Alternatively, the bias generator 86 may be activated to attract theionized plasma species electrodynamically towards the substrate supportmember. In some such embodiments, the open plasma source chamber 100′may be formed integrally with the process chamber 12 and is thereforeeffectively positioned within the process chamber 12.

[0071] A variation is shown in FIG. 6(b) where flow ports 182, 184, 186,and 188 are positioned at corners on an underside of the double-L-shapedplasma source chamber 100″. The upper portion of the figure is aperspective representation of the plasma movement within the processchamber 12, which is also shown in the lower portion of the figure as anorthographic projection of the underside. Precursor gases are providedto the open plasma source chamber 100″ with gas-delivery line 134, whichis connected to the open plasma source chamber 100″ with gas-tightconnector 132. Gas-tight connectors are also used to connect the flowports 182, 184, 186, and 188 to the process chamber 12. A plurality ofoutput flows are directed through individual segments of the rectangularstructure with inductors 190, each of which generates a component of thetotal electric field within the plasma source chamber 100″ and therebyto directs the charged plasma species.

[0072] In the specific embodiment shown, plasma loop 164 flows out flowport 182 and into flow port 184; plasma loop 166 flows out flow port 184and into flow port 186; plasma loop 168 flows out flow port 188 and intoflow port 186; and plasma loop 170 flows out flow port 188 and into flowport 182. By superposing these flows it is evident that the net flow atflow ports 182 and 186 vanishes, with a net flow into flow port 184 andout of flow port 188. In certain embodiments, this configuration maythus also be used to provide movement of plasma proximate a substratesupport holder 74. In the same manner as illustrated in FIG. 6(a), abias generator may be activated to attract the ionized plasma specieselectrodynamically towards the substrate support member. Since theplasma source chamber 100″ is integrally connected with the processchamber in this embodiment, it may be considered as effectivelypositioned within the process chamber 12. It will also be understoodthat various other positions for flow ports and plasma flow combinationsremain within the scope of the invention.

[0073] Still a further configuration is shown in FIG. 6(c) usingmultiple open plasma source chambers 100′, each in approximately theform illustrated for a single open plasma source chamber 100′ in FIG.6(a). Each of the open plasma source chambers 100′ is connected to agas-delivery line 134 through a gas-tight connector. Each also includesan inductor 192 to create an electric field within the open plasmasource chambers 100′ for directing movement of ionic species as desired.As shown in the figure, each individual chamber 100′ contributes to anoverall plasma flow in the center of the arrangement with input flowsoccurring at the circumference of the arrangement. Although notexplicitly shown for convenience of illustration, a bias generator maybe activated in the same manner as in FIG. 6(a) to attract the ionizedplasma species electrodynamically towards the substrate support member.

[0074] The use of multiple open plasma source chambers 100′ contributingflows constructively permits an increase in the overall plasma flow forapplications in which such increased flow is beneficial or desirable.While the illustrated configuration shows four approximately regularlyspaced open plasma source chambers 100′, it will be understood thatdifferent numbers of the chambers and different arrangements, includingirregular spacing arrangements, may be used to achieve particular plasmaflow characteristics. In addition, the individual plasma source chambers100 in the arrangement may, in one embodiment, be run at different RFfrequencies to limit crosstalk among them.

[0075] Having described several embodiments, it will be recognized bythose of skill in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. For example, embodiments having a greaternumber of dielectric breaks may be used. Accordingly, the abovedescription should not be taken as limiting the scope of the invention,which is defined in the following claims.

What is claimed is:
 1. A toroidal plasma source comprising: (a) ametallic plasma source chamber defining an interior for plasmageneration, the plasma source chamber including at least one dielectricbreak; (b) a drive inductor configured such that the metallic plasmasource chamber is positioned between loops of the drive inductor; and(c) an input coil configured proximate the drive inductor to provide amutual inductance between the input coil and the drive inductor.
 2. Thetoroidal plasma source according to claim 1 wherein the interior of theplasma source chamber defines a closed loop.
 3. The toroidal plasmasource according to claim 1 wherein the plasma source chamber comprisestwo dielectric breaks.
 4. The toroidal plasma source according to claim1 wherein the plasma source chamber comprises two L-shaped portionsassembled to form a rectangularly shaped enclosure.
 5. The toroidalplasma source according to claim 1 wherein the plasma source chamber ismade of a material that comprises aluminum.
 6. The toroidal plasmasource according to claim 1 further comprising means for water-coolingthe plasma source chamber.
 7. The toroidal plasma source according toclaim 1 wherein the plasma source chamber includes a liner formed on asurface in the interior of the plasma source chamber.
 8. The toroidalplasma source according to claim 7 wherein the liner is formed ofquartz.
 9. The toroidal plasma source according to claim 1 furtherincluding an RF power source capacitively coupled with the driveinductor.
 10. The toroidal plasma source according to claim 9 whereinthe RF power source is configured to operate at a frequency greater than400 kHz.
 11. The toroidal plasma source according to claim 10 whereinthe RF power source is configured to operate at a frequency ofapproximately 13.56 MHz.
 12. The toroidal plasma source according toclaim 1 wherein the drive inductor comprises two turns.
 13. The toroidalplasma source according to claim 1 wherein the input coil comprises asingle input loop.
 14. The toroidal plasma source according to claim 1wherein the metallic plasma source chamber is grounded.
 15. A toroidalplasma source comprising: (a) a plasma source chamber defining aninterior for plasma generation; and (b) a quartz liner configured toline the interior of the plasma source chamber.
 16. A toroidal plasmasource comprising: (a) a grounded metallic plasma source chamberdefining an interior for plasma generation, the plasma source chamberincluding two L-shaped aluminum portions assembled to form arectangularly shaped enclosure; (b) a quartz liner configured to linethe interior of the plasma source chamber; (c) a drive inductorconfigured such that the metallic plasma source chamber is positionedbetween loops of the drive inductor; (d) an input coil configuredproximate the drive inductor to provide a mutual inductance between theinput coil and the drive inductor.; and (e) an RF power sourcecapacitively coupled with the drive inductor.
 17. A substrate processingsystem comprising: (a) a process chamber; (b) a substrate support withinthe process chamber and disposed to hold a substrate; and (c) a toroidalplasma source configured to provide plasma to the process chamber, thetoroidal plasma source including: (i) a metallic plasma source chambercommonly grounded with the process chamber, the plasma source chamberdefining an interior for plasma generation and including at least onedielectric break; (ii) a drive inductor configured such that themetallic plasma source chamber is positioned between loops of the driveinductor; and (iii) an input coil configured proximate the driveinductor to provide a mutual inductance between the input coil and thedrive inductor.
 18. The substrate processing system according to claim17 wherein the interior of the plasma source chamber defines an openpath.
 19. The substrate processing system according to claim 17, whereinthe interior of the plasma source chamber defines an open path and thetoroidal plasma source further includes: (iv) a plurality of plasmaoutput ports configured approximately perpendicular to the closed path;and (iv) a plurality of induction coils configured to direct plasmamovement from the plasma output ports.
 20. The substrate processingsystem according to claim 12, the substrate processing system comprisinga plurality of such toroidal plasma sources, wherein such toroidalplasma sources are configured to provide plasma movement to the processchamber constructively with one another.
 21. A method for generating aplasma, the method comprising: (a) flowing a precursor gas mixture intoan interior of a grounded metallic plasma source chamber, the plasmasource chamber including at least one dielectric break; (b) inductivelycoupling an input coil with a drive inductor configured such that themetallic plasma source chamber is positioned between loops of the driveinductor; and (c) providing an RF voltage supply to the input coil toinduce an RRF electric field within the interior of the plasma sourcechamber.
 22. The method according to claim 21 wherein the plasma sourcechamber comprises two L-shaped portions assembled to form arectangularly shaped enclosure.
 23. The method according to claim 21wherein the plasma source chamber includes a liner formed on a surfacein the interior of the plasma source chamber.
 24. The method accordingto claim 23 wherein the liner is formed of quartz.
 25. The methodaccording to claim 21 wherein the RF field has a frequency greater than400 kHz.
 26. The method according to claim 21 wherein the precursor gasmix does not comprise an inert gas.